The invention of this divisional application relates to a new and improved method of cooling a continuously cast strand, particularly castings having a substantially rectangular cross-sectional configuration, and especially a steel strand.
Continuous casting operations employ the technique of casting liquid metal into a cooled open-end mold, also known in the art as a continuous casting mold, and the cast strand formed therein is continuously withdrawn therefrom. After the casting has been withdrawn from the continuous casting mold, it has not as yet completely solidified, therefore additional heat must be removed at a secondary cooling zone. During the casting of strands of rectangular cross-section, for example blooms or slabs, it is conventional practice to spray water, functioning as a liquid coolant, in the form of flat spray patterns or fans onto the surface of the continuously cast strand. A common practice in the art is to arrange a plurality of adjacently situated spray nozzles having a flat spraying characteristic in such a manner that the spray patterns or fans emanating from neighboring spray nozzles slightly overlap one another in order to strive to attain uniform cooling across the width of the continuously cast strand. This prior art arrangement of spray nozzles, which aims at providing a relatively uniform water distribution across the width of the continuously cast strand, can only operate over small transverse widths of the strand. As a result, only discrete surface portions across the width of the strand can be impinged by a single spray pattern or fan. Consequently, it is a requirement of this type of cooling system that a plurality of nozzles be arranged in respective rows across the width of the strand.
Another cooling arrangement of the state-of-the-art contemplates the use of only a single spray nozzle which is intended to spray the liquid coolant over the complete width of a slab. A decisive drawback of this arrangement resides in the fact that owing to the characteristics of the conventionally employed nozzles, the density of the spray water and thus the cooling effect at the central region of the slab is much greater than at the outer regions or portions. Additionally, the impingement forces are not uniform and the actual area of impingement by the spray pattern follows the course of a curved line or arc extending across the width of the slab. Moreover, the spray pattern or fan is not sharply defined, in fact, is unstable as the pressure varies. For these reasons, the succession of rollers which serve to guide and support the slab during its movement through the secondary cooling zone are detrimentally impinged by the spray patterns, causing uncontrollable cooling since a disturbing or interfering action is exerted upon the spray patterns. In an attempt to overcome these notable drawbacks, it has been proposed to provide a greater spacing between the successive guide rollers. As a practical matter, this is not readily possible because, due to the increased roller spacing and with casting conditions where high casting speeds are required, it has been found that the continuously cast strand, which has a relatively thin solidified outer layer around a liquid crater, tends to undesirably bulge.
A further drawback found to exist in the cooling systems heretofore proposed, resides in the tendency of the nozzles which were heretofore employed to become clogged due to the accumulation of particles which are present in the liquid coolant, typically cooling water. This again produces a non-uniform cooling effect upon the strand and also demands periodic cleaning of the nozzles, with the resultant undesirable downtime of the casting equipment and loss of production.
Modern steel casting plants must be extremely versatile in operation and capable of producing a wide range of slab sections and qualities which, in turn, requires variations of the casting speed. Metallurgical considerations make it incumbent to adapt the quantity of sprayed cooling water to the amount of heat intended to be removed within the secondary cooling zone, that is to say, as a function of the casting speed. The amount of cooling water is controlled by the water pressure prior to entering the relevant spray nozzle. It is also desirable to maintain the distribution of the spray water as constant as possible. The nozzles of conventional design heretofore employed in the cooling systems of the prior art continuous casting plants possess the drawback that as the pressure of the coolant varies, the spray water distribution also changes considerably and to a certain extent also the spray angle. Consequently, this again causes uncontrollable cooling of the continuously cast strand.